Facilitating computer-aided detection, comparison and/or display of medical images by standardizing images from different sources

ABSTRACT

The present invention is methods for processing medical images so as to remove certain effects of the physical characteristics of the object being imaged and/or of the apparatus used to form the images. The invention further provides for the formation of a standardized image from the processed image and for the use of the standardized image or the processed image in the training of computer-aided detection/diagnosis algorithms. These algorithms may then be used to detect abnormalities in other standardized or processed images derived from any of a variety of image acquisition systems.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of 10/622,978, file Jul. 18, 2003,now U.S. Pat. No. 7,680,315 which is a continuation-in-part ofapplication Ser. No. 09/992,059, filed Nov. 21, 2001 now U.S. Pat. No.7,054,473 for “A Method and Apparatus for an Improved Computer AidedDiagnosis System,” and application Ser. No. 10/079,327 filed Feb. 19,2002, now U.S. Pat. No. 7,072,498 for “A Method and Apparatus forExpanding the Use of Existing Computer-Aided Detection Code” whichapplications are incorporated herein by reference.

The subject matter of the present application is related to commonlyassigned U.S. Pat. No. 7,668,358 for “Model-Based Grayscale Registrationof Medical Images”. This application is incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to facilitating computer-aided detection,comparison and/or display of medical images. A particularly usefulapplication of the invention is in the field of radiographic mammographyand the invention will be described in detail in that context. Theinvention may also be practiced in numerous other contexts.

BACKGROUND OF THE INVENTION

Mammography is a specialized form of radiography designed to detect thesubtle changes in x-ray attenuation that are caused by cancerous tissuewhen x-rays irradiate the human breast. While there is presently nomeans for preventing breast cancer, early detection of the diseaseprolongs life expectancy and decreases the likelihood of the need for atotal mastectomy. Radiographic mammography is currently the most commonmethod of detecting and analyzing breast lesions. The American MedicalAssociation, The American Cancer Society, and the American College ofRadiology recommend yearly mammograms for women beginning at age 40.

Particular features in mammograms that are indicative of breast cancerinclude spiculated, or stellar-shaped, lesions and microcalcifications.While both features have a relatively high probability of beingmalignant, both features are also difficult to detect. To detect anyfeature at all, the x-ray attenuation of that feature must differappreciably from that of its environment. In the case of canceroustissue, there is very little difference in attenuation between canceroustissue and the glandular tissue in which it is found at x-ray energiesabove about 35 keV and a difference of only about 10% at x-ray energiesof about 20 keV. Detection of spiculated masses is further complicatedby the presence in typical mammograms of myriad lines corresponding tofibrous breast tissue. In the case of microcalcifications, while theyare almost radiopaque, they are usually very small and faint in amammogram and it is very difficult to distinguish cancerousmicrocalcifications from numerous other artifacts that are of similarsize and appearance.

A typical analog or film-based mammography system 100 is shown inFIG. 1. The system comprises an x-ray tube 110, upper and lowercompression paddles 130,135 an anti-scatter grid 140, a screen-film unit150, and a phototimer detector 160, all of which are mounted on a frame170. The x-ray tube comprises a cathode 112, an anode 114 that ismounted on a shaft 116 and rotated by a motor 118, a tube port 120, afilter 122 and a collimator 124. The screen-film unit includes an x-rayfilm 152 and a fluorescent screen 154. Phototimer detector 160 measuresthe total exposure. A control system (not shown) controls the operationof the x-ray tube including the peak voltage and tube current andterminates operation when a desired exposure as measured by detector 160has been reached. Advantageously, the control system also includes asubsystem for measuring the space between compression paddles 130, 135and therefore the thickness of the breast.

To make a mammogram, a patient's breast is compressed between a lowersurface 132 of the upper compression paddle 130 and an upper surface 137of the lower compression paddle; and the x-ray tube is turned on. Motor118 rotates anode 114 while high energy electrons bombard the rotatinganode causing the anode to emit x-rays. Some of the x-rays are emittedthrough tube port 120 in the direction of the breast located between thetwo compression paddles. The x-rays are band pass filtered by filter 122to eliminate x-rays of especially high or low energies and arecollimated by collimator 124 to eliminate those x-rays traveling inunwanted directions. The remaining x-rays pass through the breast wherethey are selectively attenuated and are incident on the anti-scattergrid 140. The x-rays that pass through the anti-scatter grid then passthrough the x-ray film with little interaction with the film and areincident on the fluorescent screen 154. The x-rays interact with thefluorescent material in the screen, causing this material to emitradiation that interacts with the x-ray film to produce the x-ray image.Some of the x-rays also pass through the fluorescent screen and areincident on the phototimer detector 160.

A variety of choices are available in the physical properties of thesesystems. The optimal x-ray energy range for these systems is about 17 to23 keV. Within this range, molybdenum has characteristic x-ray peaks at17.5 and 19.6 keV and rhodium has such peaks at 20.2 and 22.7 keV; andanodes made of one or the other of these elements are widely used.Typically, the tube port is made of beryllium which has low attenuation.The filters are typically made of the same material as the anode but arhodium filter is also used with a molybdenum anode for imaging thickerand denser breasts.

Ideally, the mammography system forms on film 152 a projection image ofthe attenuation of x-ray photons that traveled on straight lines fromthe anode through the breast to the film. However, the distribution ofphotons incident per unit area on the film is not uniform. Absorption ofphotons within the anode creates a “heel effect” as a result of whichthe area of the film directly under the anode will receive significantlyfewer photons per unit area than the area of the film under the cathode.

The photons may also be redirected by Compton or Rayleigh scattering andarrive at the film from many different angles other than anglescorresponding to a straight line from the anode. Such scattered photonsreduce the contrast in the mammogram. The amount of scatter inmammography varies with increasing breast thickness and breast area. Fora typical 5 cm-thick breast, the contrast reduction due to scatter is onthe order of 33%. To reject scatter, parallel linear grids with a gridratio of 4:1 to 5:1 are commonly used. While the film is exposed, thegrids are oscillated over a short distance to blur the grid lines. Acellular grid structure is also used in some systems to reject scatterin two dimensions.

Various screen film systems are available from suppliers such as Agfa,Fuji and Kodak. All of these systems have a single gadolinium oxysulfidephosphor screen which produces green light and a green-sensitive singleemulsion film. A variety of different speed films are available; and thecharacteristic curves of optical density versus exposure of these filmscan be quite different. Typical characteristic curves for twoscreen-film systems are shown in FIG. 2. Of particular note, thecontrast in an image is a function of the slope of the characteristiccurve.

Recently, mammography systems have become available that use digitaldetectors in place of a screen-film system. These systems producedigital mammograms without the intervening steps of processing a filmand then digitizing it. The digital systems introduce considerably morevariability in the process conditions. In addition to replacing thescreen film combination, they also use different anode targets(typically, tungsten) and possibly other filters.

In addition to variations in the physical properties of the mammographysystem, numerous operational parameters are within the control of theoperator. These include the x-ray energy, typically specified in peakvoltage (kVp), the exposure, typically specified in milli-Ampere-seconds(mAs), and the processing of the x-ray film. Another factor that clearlyaffects the optical density recorded on the film is the thickness of thebreast being x-rayed and its density (or proportion of glandular tissueto total breast thickness). To a limited degree, the thickness of thebreast being x-rayed can be controlled by the operator by adjusting thepressure exerted by the upper compression paddle.

Despite the large number of physical and operational variables thatexist in mammography systems, these differences are not an issue whenreading a single set of mammograms taken at the same time under the sameconditions. In reading the mammograms, the radiologist's attention isfocused on the relative difference between adjacent regions of themammogram; and since the mammogram was made under one set of conditions;these conditions have little effect on relative differences. However,the radiologist frequently wants to compare one set of mammograms withanother set of mammograms, for example, a set of mammograms taken theprevious year for the same person. In this case, there may besubstantial differences between the two sets, for example, because theywere taken on different systems, or recorded on different films, ortaken with x-rays of different energy, or for different exposures.Needless to say, there are also substantial differences betweenfilm-based mammograms and digital mammograms. Similar issues arise inanalyzing mammograms of different persons.

Efforts have been made to address these problems by abstracting out atleast some of the differences attributable to the physical andoperational variables. In Mammographic Image Analysis (Kluver 1999),Ralph Highnam and Michael Brady describe how to correct and remove theeffects of x-ray scatter, x-ray energy (kVp), exposure (mAs) and breastthickness. See also, their PCT application PCT/GB00/00617 filed Feb. 21,2000 and published as publication WO 00/52641 on Sep. 8, 2000, which isincorporated herein by reference. The result is a completely physicaldescription of the breast in terms of thickness and type of material—fator glandular tissue. Their interest is in the glandular or interestingtissue and they call this description H_(int), which is expressed inunits of centimeters. A complete physical description of the breastwould require a combination of H_(int) and either the total breastthickness, H_(tot), or the fat thickness, H_(fat), whereH_(tot)=H_(int)+H_(fat).

However, the H_(int) image is very difficult for the radiologist tointerpret since it is radically different from the conventional imagethe radiologist has been trained to interpret. Moreover, thecomputations needed to produce the H_(int) image are extensive andrequire substantial amounts of processing time.

Another approach is described in U.S. Pat. No. 6,516,045 for “Device andMethod for Determining Proportions of Body Materials”, which isincorporated herein by reference. As shown in FIG. 3, in this technique,two right-angled wedge-shaped reference materials 302, 304 arepositioned alongside the breast 306 between the compression paddles. Onewedge has the attenuation characteristics of fat. The other wedge hasthe attenuation characteristics of glandular tissue. The base of eachwedge is the thickness of the breast. As a result, when the mammogram isformed, an image is created of the wedges as well as the breast and theoptical density of the image of the wedges ranges continuously from avalue corresponding to 100% fat to 100% glandular tissue. Since theshape of the wedges is known, the optical density of each point in theimage of the wedges can be associated with a specific percentage of fatand glandular tissue. Then by matching each pixel of the breast imagewith the pixels of the wedge having the same optical density, thepercentage of fat and glandular tissue at that pixel in the breast imagecan be determined.

Still another approach is described in the co-pending applications Ser.No. 09/992,059 for “A Method and Apparatus for an Improved ComputerAided Diagnosis System,” and Ser. No. 10/079,327 for “A Method andApparatus for Expanding the Use of Existing Computer-Aided DetectionCode” of which the present application is a continuation-in-part. Inthose applications, various normalization techniques are described toremove the differences caused by different detectors. In particular, theapplications describe a variety of techniques for equalizing thecontrast response in which analytic expressions or “look-up” tables aredeveloped that convert the response measured by one system to what theresponse would be if measured by another system. While these techniquesfacilitate the analysis and comparison of mammograms made usingdifferent detectors, they do not address differences arising fromdifferent exposure parameters or differences in breast thickness.

Another area in which it would be advantageous to compensate fordifferences arising from different exposure parameters or differences inbreast thickness is in the development of algorithms for computer aideddetection and diagnosis of abnormalities in medical images such asmammograms.

The algorithms that are presently used are heavily dependent on thetraining of the algorithms using groups of mammograms. See, for example,U.S. Pat. No. 5,491,627 to Zhang et al., U.S. Pat. No. 6,075,879 toRoehrig et al., and the above-referenced Ser. No. 10/079,327 for “AMethod and Apparatus for Expanding the Use of Existing Computer-AidedDetection Code.” At present, to obtain a sufficient number of mammogramsfor training purposes, the set of training mammograms includesmammograms formed on different mammographic systems. As a result, theperformance of the computer aided detection system is not as great as itwould be if the training had been performed on the same sized set ofmammograms made on a single system. It would be advantageous to be ableto train the detection algorithm using larger sets of mammograms made oneffectively the same mammographic system.

SUMMARY OF THE INVENTION

The present invention is methods for processing medical images so as toremove certain effects of the physical characteristics of the objectbeing imaged and/or of the apparatus used to form the images. Theinvention further provides for the formation of a standardized imagefrom the processed image and for the use of the standardized image orthe processed image in the training of computer-aideddetection/diagnosis algorithms. These algorithms may then be used todetect abnormalities in other standardized or processed images derivedfrom any of a variety of image acquisition systems.

BRIEF DESCRIPTION OF THE DRAWING

These and other objects, features and advantages of the invention willbe more readily apparent from the following detailed description of theinvention in which:

FIG. 1 is a schematic illustration of a prior art analog mammographysystem;

FIG. 2 is a depiction of the characteristic response of two radiographicmammography films;

FIG. 3 is a schematic illustration of a prior art modification to amammography system;

FIG. 4 is a flowchart depicting a first embodiment of the invention;

FIG. 5 is a flowchart depicting a second embodiment of the invention;

FIG. 6 is a flowchart depicting a third embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 4 is a flowchart depicting a first embodiment of the invention. Atstep 410 a mammogram is formed using either an analog mammography systemsuch as that shown in FIG. 1 or a digital mammography system. If ananalog system is used in step 410, the resulting mammogram film is thenscanned at step 415 to convert the analog image into a digitized image.At step 420, the digital image formed in step 410 or the digitizedimaged formed in step 415 is processed to remove at least one andpreferably substantially all distinguishing effects related to thephysical characteristics of the first mammography system and itsoperating parameters.

These effects may include effects arising from physical characteristicsof the mammography system such as anode material, source to imagedistance, anti-scatter grid geometry, film characteristics, and screenfilm system. They may also include operating characteristics of themammography system that may vary from patient to patient even when usingthe same mammography system such as x-ray energy, and magnitude andduration of exposure as well as physical differences between patientssuch as thickness and density of the breast being imaged. In addition,processing preferably is performed to measure the relative fat contentof the breast being imaged.

The processing of the digital or digitized mammogram is preferablyperformed by a computer. Preferably, the processing performed isessentially the same as that detailed by Highnam and Brady in their bookMammographic Image Analysis and in their PCT patent applicationPCT/GB00/00617. As indicated above, extensive processing is necessary toremove the effects of the mammography system as well as to remove theeffects of the fat content of the breast which is not interesting to theradiologist.

The result of the processing is an image representative of the physicalcharacteristics of the breast that was imaged by the first mammographysystem and, in particular, representative of its glandular (non-fatty)tissue content. This image is referred to by Highnam and Brady as theH_(int) (or interesting) image and comprises a two-dimensional array ofnumerical values representative of the thickness of glandular tissue ateach point in the array.

At step 430, the processed image is converted from the H_(int) image toa standard-form mammogram by calculating what the original mammogramwould have looked like had it been made on a standard-form mammographysystem having a standard set of physical characteristics and a standardset of operating parameters. These calculations are essentially thereverse of the calculations used to form the processed image but usingin this case the physical characteristics and operating parameters ofthe standard-form mammography system. For example, for the standard-formsystem, the x-ray energy preferably is set to 25 kVp, the exposure to100 mAs, and the breast thickness to 5 cm. These parameters arepreferred in view of the energy dependence of breast x-ray attenuation.Alternatively, the x-ray energy may be set at a value within the range25-28 kVp and the exposure at a value within the range 20-200 mAs, andthe thickness to a value within 4-6 centimeters. The processing requiredto form the standard-form image is extensive and preferably is performedby a computer.

At step 440, the standard form mammogram is then stored.

In like fashion, other digital or digitized mammograms formed by thesame mammography system or by different mammography systems aresimilarly processed by the system of the present invention to formprocessed images from which have been removed the same effects relatedto the physical characteristics and operating parameters of themammography system as well as to the fat content of the breast; and theprocessed images are converted to standard-form mammograms and stored.

The standard-form mammograms are then available to be compared at step450. In all cases, the standard-form mammograms are free of thedifferences that otherwise would be present arising from the use ofdifferent mammography systems and/or different operating parameters inthe formation of the mammogram. In the case where the standard-formmammograms are images of the same breast, the images will be essentiallythe same except for any physiological changes that have occurred in thebreast between the time the first mammogram was taken and the time anylater mammogram was taken.

Comparisons may be made using a variety of modalities such as computerwork stations, film display systems, and images printed on paper. Again,there are considerable differences in the display characteristics of thedifferent modalities and significant differences in the displaycharacteristics of different suppliers' products in the same modality.Of particular interest are differences in brightness and contrast in thedisplayed images. Advantageously, these differences can be controlled tomeet the needs of the user. For example, analytic expressions and/orlook-up tables can be developed that relate pixel values ofstandard-form images stored at step 440 to levels of brightness on aparticular manufacturer's work station, to levels of optical density ina film or to gray scale values in an image printed on paper by a laserprinter. These expressions or look-up tables are then used at step 450to convert the standard form images to images having the desiredbrightness and contrast in the display modality of choice.

To understand how to obtain a purely physical description of the breastsuch as the H_(int) image, a simplified example of computing H_(int) ispresented here for three hypothetical locations in a breast, including afirst location that is 100% fat, a second location that is 50% fat and50% glandular, and a third location that is 100% glandular. We willassume the breast is uniformly exposed to mono-energetic x-rays in adevice such as shown in FIG. 1. We will assume further that thecompression thickness H_(tot) is uniform—i.e. the compression paddlesare parallel. Actual computations to account for the true x-rayspectrum, and to correct effects such as tilt in the compressionpaddles, will be more complex than shown here, but the description belowis adequate to describe the principle of how to derive the H_(int) imagefrom the exposure of the actual breast.

The intensity of x-ray penetrating material is given by Beer's law:I _(out) =I _(in) exp(−μΔh)where μ is the attenuation of x-rays through matter, and Δh is thethickness of the material. Let us suppose the attenuation of fat isgiven by μ_(fat) and the attenuation of glandular tissue is μ_(tissue).In reality the attenuation is also a function of energy of the x-ray,but here we will assume that the x-rays are mono-energetic and theattenuation can then be represented with a single number μ.

Then the intensity after passing through H_(tot) of a fatty section, anequal proportion of fat and tissue, and a pure tissue regions will be:I _(in) exp(−μ_(fat) H _(tot))I _(in) exp(−(μ_(fat)+μ_(tissue))H _(tot)/2)I _(in) exp(−μ_(tissue) H _(tot))

We assume that the exposure parameters are somehow available so thatI_(in) (in units of mAs or milliAmp-sec) is known, the values forattenuation μ in fat and tissue are known, and the thickness H_(tot) hasbeen measured. If the mammogram has been acquired on film/screen, theseparameters are written onto the patient labels which appear on themargins of the films. Co-pending patent application Ser. No. 10/142,704for “A Method and Apparatus to Associate User Data from a Radiograph”describes an automated technique for obtaining that information usingoptical character recognition (OCR). This application is incorporatedherein by reference. If the mammogram has been acquired on a digitaldetector, the exposure parameters are entered into fields in thestandard DICOM header which is created for each image.

The equations above relate the intensity of x-rays at the detector tothe exposure and material characteristics. To actually perform anyactual operations in the computer, we must also know how the intensitymaps into pixel value in the computer image. This mapping is given bythe equationPV=K ln(I/I′),which is the linear portion of the film response shown in thecharacteristic curves of FIG. 2. For purposes of this discussion,without loss of generality, let us ignore the constants K and I′. Thenthe pixel values for the image of the fatty section, an equal proportionof fat and tissue and a pure tissue region are:PV ₁=ln(I)−μ_(fat) H _(tot)PV ₂=ln(I)−μ_(fat) H _(tot)/2−μ_(tissue) H _(tot)/2PV ₃=ln(I)−μ_(tissue) H _(tot)Or, subtracting the constant factor ln(I)−μ_(fat)H_(tot), and definingμ_(Δ)=μ_(tissue)−μ_(fat), we can express the three pixel values as:PV1=0PV2=−μ_(Δ) H _(tot)/2PV3=−μ_(Δ) H _(tot)which are in the form PV=μ_(int)H_(int), where H_(int) is 0 in theportion that is 100% fat, ¼ of H_(tot) in the region with 50% tissue,and 1H_(tot) in the region with 100% tissue. Thus, the pixel values area function of the thickness of the glandular or interesting tissue inthe breast. This is essentially the “H_(int)” image as used by Highnamand Brady, and provides a representation of the mammogram which is freeof the exposure parameters kVp, mAs, and compression thickness.

The standard image formed at step 430 differs from the H_(int) imagedescribed in the above in that it will have the appearance of an x-raymammogram with a known, “standard” thickness. Without loss ofgenerality, we choose 5 cm as the “standard” thickness because this is athickness that is quite common in actual mammograms. Further, we assumethe mono-energetic x-ray energy to be 25 kVp, and the intensity to beI=100 mAs.

We convert the H_(int) image to the standard-form “normal” image asfollows. The minimum pixel value of this particular H_(int) image is 0,which is the assigned value for pure fatty region of thickness 5 cm.Given a value of attenuation for fat at 25 kVp to be μ_(fat(25)), theH_(int) image can be converted to a standard form image by adding ln100exp[μ_(fat(25)*()5−H_(int))] to each pixel value. The mapping of theH_(int) image to the standard form image expressed by this exponentialcan be implemented in a look up table (LUT). If it is desired torepresent the image as it would appear with other exposure parameters,i.e., other H_(tot), intensity I, and attenuation corresponding toanother kVp, this mapping can be implemented by:PV _(new) =PV _(norm)+[ln(I _(new))−μ_(fat)(kVp)T],which can be implemented in another LUT.

It remains to show how to map the appearance into the appearance of thestandard form image of any given detector.

FIG. 2 shows the characteristic curves of two film brands used inmammography—Kodak Min-R2000 and Agfa HDR. These can be obtained from themanufacturers, or measured empirically by, for example, step wedges. Themapping from pixel value obtained from Agfa, for example, to KodakMin-R2000 can be obtained directly from the two curves in FIG. 2, whichcan be implemented easily in a LUT. For example, an OD of 1.6 on Agfawould be mapped to approximately 2.4 to convert to a “Kodak” appearance.Similarly, a film/screen image from e.g., Kodak, can be converted to adigital detector which also has a characteristic response. Conversionfrom all types of mammographic image into all other types can beimplemented in look up tables. Hence, we can in principle convertmammographic images from all sources into the common form and from thecommon form into any display form thus removing dependence on detectortype.

FIG. 5 is a flow-chart depicting a second embodiment of the invention.At step 510 a mammogram is formed using either an analog mammographysystem or a digital mammography system. Two wedge-shaped materials asillustrated in FIG. 3 are irradiated at the same time as the breast andtheir images are formed alongside the breast on the mammogram. If ananalog mammography system is used in step 510, the mammogram film isthen digitized at step 515 by a scanner to produce a digitized image ofpixel values.

Whether formed directly by a digital mammography system in step 510 orby digitizing a file in step 515, the portion of the resulting imagethat is the image of the wedges will have pixel values ranging from afirst value corresponding to 100% fatty tissue to a second valuecorresponding to 100% glandular tissue; and each pixel value in theimage of the breast between the first and second values will beassociable with a specific percentage of fatty and glandular tissue.

At step 520, a look-up table is created associating each pixel value inthe image of the wedge with a specific percentage of fat and glandulartissue; and the pixel values in the digital or digitized image of thebreast are converted to an image representative of the physicalcharacteristics of the breast (i.e., percentage of fatty and glandulartissue) by using the table to convert pixel values in the breast imageto percentages.

The physical image may then be used in the same fashion as the processedimage produced at step 420 in the first embodiment. In particular, thephysical image may be converted at step 530 to a standard form mammogramusing a standard set of exposure parameters such as a peak energy levelof 25 kVp and an exposure of 100 mAs for a breast having a thickness of5 cm. The standard form mammogram may then be stored at step 540.Additional standard form mammograms may likewise be made and stored andany of these mammograms may later be retrieved at step 550 for displayand comparison on suitable display modalities.

In addition to facilitating the comparison of mammograms, the presentinvention also facilitates the analysis of mammograms in computer aideddetection and diagnosis systems. As indicated above, the algorithms usedin such systems are typically trained on a large number of mammogramsand this typically means that the mammograms were made on more than onemammography system.

In accordance with another embodiment of the present invention,algorithms are trained either on standard-form images or on physicalimages and mammograms are analyzed by applying the algorithm to eitherthe standard-form image or the physical image of the mammogram.

FIG. 6 is a flowchart depicting this embodiment of the invention. Atstep 610, a group of mammograms that have been converted to standardform images by the method of FIG. 4 or by the method of FIG. 5 are usedto train a computer aided detection or diagnosis (CAD) algorithm.Advantageously, this group of mammograms is relatively large and isderived from a multiplicity of different mammography systems. Forexample, the group may include 100 mammograms made on each of fourdifferent systems. However, because the differences in the mammogramsarising from differences in the different mammography systems have beenremoved as a result of the methods of FIG. 4 or FIG. 5, the group ofmammograms appears to the training algorithm as one large set ofmammograms from the same system. At step 620, the CAD algorithm isstored.

At step 630 the CAD algorithm is used to analyze a mammogram that hasbeen made on any mammogram system and then converted to a standard formmammogram.

Alternatively, the training of the CAD algorithm could be performed onthe physical images of the group of mammograms formed at step 420 orstep 520 in which case the analysis of the mammogram would be performedon its physical image.

By using either standard form mammograms or physical images to train theCAD algorithm and then analyzing the standard form mammogram or physicalimage, only one CAD algorithm is needed. Moreover, a much larger numberof mammograms are available to train the CAD algorithms.

FIG. 7A is a reproduction of a mammogram formed by a conventionalmammography system. FIG. 7B is a reproduction of a mammogram formed bythe method of FIG. 5 and converted to a standard form. The image of FIG.7B has been processed to minimize the effect of any fat content in thebreast image. A comparison of regions 710A and 720A in FIG. 7A withregions 710B and 720B in FIG. 7B reveals that features that areessentially indistinguishable in regions 710A and 710E are readilyrevealed in regions 720A and 720B.

1. A method for processing mammographic images comprising the steps of:processing a plurality of digital or digitized mammograms formed bydifferent x-ray mammography systems to remove distinguishing effects ofeach mammography system; transforming, with a computer, each processedmammogram into a standard-form x-ray mammogram having a first standardx-ray voltage parameter and a first standard exposure parameter, whereinat least one of the first standard x-ray voltage parameter and firststandard exposure parameter differs from at least one of an x-rayvoltage parameter and exposure parameter of at least one of the digitalor digitized mammograms; storing said standard-form x-ray mammogramswhereby visual comparison of x-ray mammograms taken by different x-raymammography systems is facilitated by comparing standard-form x-raymammograms derived from mammograms taken by the different x-raymammography systems.
 2. The method of claim 1 wherein the processingremoves distinguishing effects of at least one of an x-ray voltage, anexposure voltage and a compression thickness.
 3. The method of claim 1wherein an x-ray image of a reference material and one of the mammogramsare formed under a set of conditions, said reference material havingknown x-ray attenuation characteristics representative of differentpercentages of fat content, said method further comprising the step ofidentifying fat content in the one of the mammograms by comparingexposure values in the one of the mammograms with exposure values on thex-ray image of the reference material.
 4. The method of claim 1 whereinthe standard x-ray voltage parameter is in the range 25-28 peak voltage(kVp).
 5. The method of claim 1 wherein the standard exposure is in therange 20-200 milli-Ampere-seconds.